1. Technical Field
The disclosed embodiments relate to low-noise SAW-less receivers having passive mixers.
2. Background Information
There are several different circuit topologies for realizing a cellular telephone receiver. Some topologies involve a surface acoustic-wave (SAW) filter that is disposed in the incoming RF signal path between a low-noise amplifier (LNA) and a mixer. It is desirable to be able to realize a cellular telephone receiver of adequate performance without using the somewhat costly and large SAW device. Other topologies use a high quality and somewhat expensive LNA matching inductor in the matching network between the antenna and LNA. It is also desirable to realize the receiver without using this expensive component, but rather by using less expensive low quality components. In order to achieve good linearity in a SAW-less receiver, a purely passive down-converting mixer is sometimes used. The passive mixer does not have an active gm-cell. It is, however, often difficult for this topology to meet demanding noise figure (NF) requirements while still achieving good linearity (e.g., triple beat) performance.
FIG. 1 (Prior Art) is a diagram of a SAW-less receiver circuit 1 that employs a passive mixer and no active gm-cell. Receiver circuit 1 includes an antenna 2, a differential duplexer 3, an impedance matching network 4, and an integrated circuit 5. Integrated circuit 5 in turn includes an LNA 6, and this LNA is coupled to receive an incoming RF signal from matching network 4 via two terminals 7 and 8. Two differential output leads 9 and 10 of LNA 6 are coupled to the passive mixer 11. The coupling of LNA 6 to mixer 11 may be a capacitive coupling via bypass capacitors having large capacitances that are effective shorts at the frequencies of incoming RF signals.
Passive mixer 11 has two portions. One portion (I-path switching core) supplies a first set of differential signals (I) to a first part of a transimpedance amplifier and filter 12 (I-path TIA). This first part of TIA 12 outputs signals I+ and I− onto output leads 13 and 14. The second portion of passive mixer 11 (Q-path switching core) supplies a second set of Q differential signals to a second part of TIA 12 (Q-path TIA). The second part of TIA 12 outputs the signals Q+ and Q− onto output leads 15 and 16. Unfortunately, it is often difficult to employ the SAW-less passive mixer receiver topology of FIG. 1 and also satisfy stringent noise requirements, while at the same time maintaining good linearity and power consumption performance.
FIG. 2 (Prior Art) is a table that sets forth various performance characteristics of the circuit of FIG. 1. It is desired to reduce the noise figure (NF) of the circuit without providing a high-quality and expensive discrete LNA input matching inductor in the matching network 4 and without increasing the current consumption of LNA 6.